I just thought of two guys that do great American accents. Just thinking out loud but maybe the fact that they are black might subconciously help when an American hears them. I know Britain is multicultural but black is not the first thing that pops into my head when thinking “British”.
Eamon Walker who I know mostly from the HBO series Oz. A flawless accent in my opinion.
Lennie James from Jericho is from London but I couldn’t tell until I looked him up.
I was gonna mention Walker! I saw him on a UK show and I was like, “Is that Said? No way!”
The bloke who plays Carcetti on The Wire - Aidan somebody - is at times good, as cited upthread, but this past season he lost it and became this nasally Irish-American-sounding accent at times.
I think Hugh Laurie is great. Listen to him play an American from his sketch days on A Bit of Fry and Laurie - cringeworthy there, quite good now.
I also think Rachel Griffiths did great on Six Feet Under.
The guys who did the voices on Spitting Image in the 80s - including Harry Enfield - did great accents. Enfield is so good, most people didn’t know his annoying “Dr. Angus” character from the Burger King commercial wasn’t American. His sketches have very good, middle-American accents. His last series with Paul Whitehouse was alternately brilliant (his accent) and hideous (Whitehouse’s).
Bad accents:
Tracey Ullman can definitely go overboard. Most of the time she’s good, but other times not so much. Her Three Of A Kind costar, Lenny Henry, does a funny (but pretty bad) American accent.
Daphne from Frasier is horrible. I really hated her guest spot on Seinfeld. I was gobsmacked to learn she was a Brit.
I’ve already mentioned Whitehouse. Americans won’t know him (he was a major player on The Fast Show/Brilliant!).
Americans have totally tin ears for non-American accents, which I find annoying. I have no problem understanding most Commonwealthers - Jamaicans, Aussies, and so on - but I remember being amused when Oasis were big in America, they had Noel Gallagher’s interviews with subtitles. Liam’s a bit mushmouthed but Noel is quite understandable.
According to the interview at the bottom of this page, it is supposed to be Mancunian. I think it is an interesting accent in that to me it is obviously Mancunian yet does not sound like any Mancunian I have ever met. It is some sort of caricature accent.
Pretty much everyone. He is most well known for musical theater. Here he is in the movie version of The Producers (musical). Here he is in the movie Delovely with Kevin Kline.
I would love to say that it is because of my vast knowledge in multiple subjects. In fact I never heard of him until Doctor Who. Then I saw him in The Producers. Then I started to look into more clips of him.
When I was an actor I, and all my actor friends, had “American accent” down in our skillset. I thought I had an advantage, having had a Texan accent between the ages of 5 and 10, and having spent a lot of time in Texas, Tennessee and Connecticut. But in about 2000, when the company I was working for needed an American accent for a voiceover and I volunteered, I could hardly listen to the playback of my own voice. I sounded like I should have been working for the BBC drama department: I had produced a lot of stereotypical American consonants, but filled them in with very clipped RP vowels. A ghastly sound.
Ah, the ambiguity of writing pronunciations in plain English. To me (and, I suspect, most Brits), “new” brings to mind the sound “nyoo”, entirely different from the “noo” in “nougat”.